1. Field of the Invention
This invention relates to switching power supplies and more specifically to switching power supplies with reduced switching losses at high frequencies.
2. Prior Art
FIG. 1 illustrates a typical prior art half-bridge switching power supply 8 which receives a DC input voltage V.sub.in across a pair of terminals 10a and 10b and generates therefrom a DC output voltage V.sub.out across a pair of output terminals 12a and 12b.
Power supply 8 includes capacitors C1 and C2, which function as a capacitive voltage divider so that the voltage at a node N1 is approximately equal to voltage V.sub.in /2. Power supply 8 also includes first and second switching transistors Q1 and Q2 which periodically turn on and off such that when transistor Q1 is on, transistor Q2 is off, and when transistor Q2 is on, transistor Q1 is off. Transistors Q1 and Q2 typically have equal duty cycles. When transistor Q1 is on, a voltage of approximately V.sub.in /2 (the voltage at lead 10a minus the voltage at node N1) is applied across primary winding P1 of transformer T1. When transistor Q2 is on, the voltage across winding P1 equals -V.sub.in /2 (the voltage at lead 10b minus the voltage at node N1). Voltage V.sub.out is generated in response to the waveform applied to winding P1 by a conventional filter and rectifier circuit comprising diodes D1 and D2, inductor L1 and capacitor C3 coupled to a secondary winding S1. A regulator circuit 14 senses voltage V.sub.out and increases the duty cycle of transistors Q1 and Q2 if voltage V.sub.out is too low, and decreases the duty cycle of transistors Q1 and Q2 if voltage V.sub.out is too high.
Diodes D3 and D4 are coupled across transistors Q1 and Q2, respectively to prevent large voltage spikes across and current spikes through transistor Q1 when transistor Q2 turns off, and large voltage spikes across and current spikes through transistor Q2 when transistor Q1 turns off.
Unfortunately, each time one of transistors Q1 and Q2 turns on and then turns off, power supply 8 consumes an amount of power known as a switching loss. As the switching frequency of transistors Q1 and Q2 increases, the amount of energy loss per unit time exhibited by power supply 8 increases, thereby making power supply 8 less efficient.
The switching loss incurred by power supply 8 is caused in part by the fact that transistors such as Q1 and Q2 exhibit capacitance across the collector and emitter. Thus, transistor Q1 can be modeled as illustrated in FIG. 2, with a capacitor C4 coupled across the collector and emitter. When transistors Q1 and Q2 are off, a voltage equal to approximately V.sub.in /2 is applied across the collector and emitter of transistor Q1. Therefore, an amount of energy is stored in capacitor C4 equal to C(V.sub.in /2).sup.2 /2, where C is the capacitance of capacitor C4. When transistor Q1 turns on, capacitor C4 is discharged through transistor Q1, and all of the energy stored in capacitor C4 is dissipated in transistor Q1. Thus, every switching cycle, an amount of energy equal to C(V.sub.in /2).sup.2 /2 is wasted by turning on transistor Q1 and by turning on transistor Q2. It would be desirable to eliminate this switching loss.
It is also known to use MOS transistors in switching power supplies, and that MOS transistors can be operated at higher frequencies than bipolar transistors, e.g. frequencies greater than 1 MHz. However, such MOS transistors exhibit capacitance across the source and drain, and therefore incur a switching loss similar to the loss described in reference to FIG. 2. At high frequencies, these switching losses, which are proportional to frequency, become so great that the power supply cannot operate efficiently.
Other types of power supplies include full bridge converters and push-pull converters, described, for example, by Chryssis in a book entitled "High-Frequency Switching Power Supplies", published by McGraw-Hill Book Company in 1984, incorporated herein by reference. Such switching power supplies also exhibit the above-described switching losses.